Elastomeric, fibre bonded or leather seals are rarely satisfactory in such conditions because it is difficult to lubricate the sliding surfaces satisfactorily and at the same time prevent the seal material being extruded into the clearance between the piston and the cylinder bore. The combination of high pressure and low viscosity tends to bring about early or even immediate failure of the seal.
Piston rings can overcome the extrusion problem, but require better lubrication than provided by the commonly used fluids in high pressure jet applications.
An alternative approach has been to use a closely fitting piston without special seals. In this method, the clearance between piston and cylinder bore has to be closely controlled and materials carefully selected to ensure adequate lubrication. With low viscosity fluids, the axial length for which the close clearance must be maintained is considerable if the leakage is to be kept within acceptable limits. The combined requirement of a very small radial clearance between piston and cylinder bore and its maintenance for a considerable length results in high costs of manufacture and bulky equipment.
A well known technique for overcoming this problem is to use a supply of good lubricating and easily sealed fluid, typically an oil, which is introduced into the machine in the vicinity of the seals so that the seals operate with this good lubricating fluid.
One form of this well known technique is to use two seals on a single piston and to feed a viscous liquid, such as oil, into the cavity between the two seals. The pressure of the oil feed is arranged to be equal to or greater than the pressure reached by the lower viscosity liquid, such as water, so that the seal separating the cavity from atmosphere only operates with oil and, as the pressure of the water in the inner part of the cylinder never exceeds the pressure of the oil in the cavity, the seal separating the cavity from the inner part of the cylinder never operates with the water and hence the oil always lubricates both seals.
The disadvantage of this method is that a separate supply of oil at a pressure equal to or greater than the output pressure of the water pump is required.
In another form of high pressure piston pump, oil is feed into the bore so that it remains in the vicinity of the seal on a main piston and consequently this seal is lubricated with oil. A subsidiary piston is used to provide a physical separator between the oil and water. The pressure drop across this separator is very small, being only that required to overcome the friction and inertia of the subsidiary piston. During the operation of the pump, oil leaks past the seal on the main piston at an indeterminate rate and provision must be made to make up this loss. The volume of oil in the bore of the pump, between the main and subsidiary pistons, needs to be controlled for satisfactory operation. If it were to become too small, the subsidiary piston would contact the main piston and the full water pressure would be developed across the separator. If it were to become too large, as a result of a reverse pressure differential across the subsidiary piston during non-delivery strokes of the main piston, the subsidiary piston would be driven to the end of the cylinder before the main piston had completed its delivery stroke.